Endogenous Pacemakers and Exogenous Zeitgebers

• Endogenous pacemakers are pacemakers within our body. The most important EP is the suprachiasmatic nuclei (SCN). The SCN is in the hypothalamus and is the main EP, the master clock. It's responsible for the control of the sleep/wake cycle and other CRs including body temp and heart rate. The SCN has a built in CR and resets when external light levels change. The SCN receives info from the optic nerve and regulates sleep by influencing activity of the pineal gland
• The SCN regulates the sleep cycle by communicating with the pineal glad. The SCN sends signals to the pineal gland by instructing it to increase secretion of melatonin at night (inducing sleep). Reduces in the morning as light increases. The pineal gland and SCN collectively control the sleep wake cycle but their activity must be sychronised with the light-dark cycles of the outside world. If the SCN isn't in synch with the outside world, it disrupts the sleep wake cycle
• EVAL points EP


  • S - EP - evidence that SCN is the main EP from animal studies
  • S - EP - support for role of EP - case studies
• Exogenous zeitgebers are influences outside the body and the most dominant is light. Light resets the internal bio clock (SCN) every 24 hrs by activating light detecting cells in the retina cells which contain melanopsin.
• The optic nerve carries the signal to the SCN which influences the pineal gland to reduce melatonin production which makes us feel awake and resetting the sleep wake cycle. Leads to entrainment of the human body alongside the environment which helps us better adapt to the environment by influencing endogenous cycles
• Other EZs which control our bio rhythms e.g. mealtime clocks noise. While light is dominant, individuals are able to compensate for absence of light by responding to social zeitgebers
• S - EP - evidence that the SCN is the main EP from animal studies. Morgan removed SCN from hamsters and found their CRs disappeared. Also when hamsters are bred with a CR of 20 hrs rather than 24 hrs, and their SCN's are then transplanted into normal hamsters, the normal hamster will display the mutant rhythm. S bc changes in the SCN lead to changes in CR. This provides evidence for the importance of the SCN in creating and maintaining CRs. Increase V of R into EP
• S - R support for the role of EZ. Burgess found that when Ps were exposed to continuous bright light right after they woke up, their CRs shifted by 2.1 hrs over the course of the study. As a result, Ps felt sleepier 2 hrs earlier in the evening and were better adjusted to the local time at their destination after a along flight. This demonstrates the role of light in CR as it shows that changes of external light patterns lead to alterations of the sleep wake cycle. This happens despite no direct changes made to EPs. Shows EZ are a key influence to sleep wake cycle. Increase V
• W - EZ - refuting evidence. Siffre spent 7 months underground in a cave. He was adequately fed, had opportunities for exercise and was able to make contact at all times by telephone but he totally lacked any cues about when it was day or night. His sleep pattern settled down to just over 24 hrs. Conc that the 24 hr cycle of sleep/wake is internally caused. W bc despite the absence of light, his CR maintained. Decreases v
• S - role of EP maintaining a 24hr cycle. Siffre spent 7 months in an underground cave. He had food and opportunities to exercise but he had no cues about whether it was day or night. He found that his sleep pattern settled around 24 hours. S bc it shows that maintenance of a 24hr sleep wake cycle is internally caused by EPs. Bc this circadian rhythm still existed in the absence of external cues, this shows that EPs are responsible for maintaining a 24hr rhythm. /V